Spark plugs for vehicle engines are designed to seal the combustion chamber so that exhaust gases cannot vent directly into the atmosphere, but instead must pass through an appropriate vehicle exhaust system.
With reference to FIGS. 1-1B, there is shown a cross-sectional view of a spark plug 10 having a conventional arrangement that includes a shell 12, insulator 14, center electrode assembly 16, and ground electrode 18. An external seal is established between shell 12 and the cylinder head (not shown) when the spark plug is installed and screwed into the cylinder head so that a conical shell seat or a separate external gasket 20 is compressed against a seat portion in the cylinder head. An internal seal, on the other hand, is established between insulator 14 and shell 12 and is typically achieved with a separate internal gasket or gasket ring 22, which is located between a seat portion 30 of the shell and a shoulder portion 32 of the insulator. According to this design, internal gasket 22 is a tapered ring that contacts seat and shoulder portions 30, 32 with side surfaces 40, 42 of the gasket, respectively, as opposed to contacting such portions with end surfaces 44, 46 of the gasket. In order to ensure that the internal seal sufficiently seals or blocks off exhaust gases that are under pressure in the combustion chamber, the insulator, gasket ring and shell are usually pre-loaded or compressed in the axial direction so that a good seal is formed. Axially or compressively pre-loading these components, however, can introduce an axial stress AS into insulator 14.
One area of insulator 14 that tends to be vulnerable to stress and breaking is the area of the insulator between positions B and C in FIGS. 1-1B. This is particularly true if the axial stress AS from the pre-loading is coupled with a radial or bending stress RS that is exerted against the insulator core nose 36 in an area between positions A and B. A potential source of the radial stress RS is a pressure wave resulting from engine knock or other misfiring events. If the overall or combined stress (e.g., stresses AS+RS) exceeds the internal strength of insulator 14, which is usually made from a somewhat brittle ceramic material, then the insulator can crack, break or otherwise fail.